How to make more then one character from one rigged mesh
Adjusting a Rigify rig is not impossible but it's quite complicated, due to the presence of lots of machinary and auxiliary bones, that can be revealed turning on all the bone layers of the armature.

The best way to achieve your result is to adjust the metarig to the new copy of the character, and generate a new rig (change the name of the first rig before generating the second).

If you don't want to redo the wheights assignement, you can use all weight already present, just bear in mind that the new rig will have a new name, and you will have to set this new name in the object field of the armature modifier.